trigger ContractAutoApprove on Opportunity (before update) {

for(Opportunity opp : Trigger.new){
   String cs = 'Contract - Submitted';
   String ca = 'Contract - Approved';
   if (Opp.StageName == cs){
     if (Opp.Contract_Discount__c == 0){
     Opp.StageName = ca;  
     }
   }
}
}